Re: How common are five year missions?
Actually, since the narration talks about the ship and not the crew (Voyages of the starship Enterprise. HER 5 year mission), five years may simply be standard time between refits. Kirk had obviously been onboard long enough to get chummy with Spock, the long mission to the edge of the galaxy and all. It may have been closer to 6 years and Kirk just rounded down in TMP.
We can admit that we're killers ... but we're not going to kill today. That's all it takes! Knowing that we're not going to kill - today! - Kirk - A Taste of Armageddon